The field of art to which this invention pertains is water reducible coating compositions and to air-drying coatings made therefrom.
Water-borne coatings are being investigated for use in various industrial applications. One field of interest is the industrial maintenance field. Industrial maintenance coatings are coatings which are generally applied as primers and topcoats to metals and which must be resistant to corrosive atmospheres and to attack by chemicals and solvents. Industrial maintenance coatings are generally applied to existing structures by spraying or brushing and, therefore, the coatings must be capable of air-drying, i.e., drying or curing at ambient temperatures. Air-drying coatings usually used as industrual maintenace coatings are those coatings which dry or cure through autoxidative polymerization. Such coatings are generally based on resins derived from drying oils or modified with unsaturated fatty acids.
Water soluble or water dispersible resins from which water reducible air-drying coating compositions have been prepared are anionic resins, i.e. resins which contain acid groups, e.g., carboxylic acid groups, in their structure and which are solubilized with a base. Other resin systems are based on nonionic resins which are emulsified in water. Water soluble and water emulsifiable resins are described in U.S. Pat. No. 4,199,487.
Other types of water soluble or water dispersible resins which have been used to make water reducible coating compositions are cationic resins, i.e., resins which contain a basic nitrogen group and which are solubilized with an acid. Paints made from such cationic resins have been found to have superior corrosion resistance when formulated into primer paints. Such paints are particularly useful in electrodeposition processes and for industrial applications. However, such cationic coating compositions require heating for cure and have not been found useful for industrial maintenance applications.
U.S. Pat. No. 3,984,299 is representative of a number of patents which describe aqueous coating compositions made from adducts of epoxy resins and primary or secondary amines, said adducts being salted with acids to render them water soluble or water dispersible. The epoxy-amine adducts are modified by esterification with carboxylic acids, especially fatty acids. The preferred fatty acids are saturated fatty acids such as pelargonic acid. These cationic coating compositions are not air-drying compositions, but are formulated into heat curable compositions by the addition of a crosslinking agent, i.e., a blocked polyisocyanate.
Other cationic resins are described in U.S. Pat. No. 4,134,864. Such resins are made by reacting an epoxy resin with a polyamine followed by amidification with a monocarboxylic acid which can be saturated or unsaturated. These cationic resins contain residual amine hydrogen atoms and are not air-drying. Coating compositions formulated from such cationic resins contain crosslinking agents, such as aminoplast resins or phenolic resins, and are cured by being heated at elevated temperatures.
Cationic resin based coating compositions are also described in copending, commonly assigned patent application entitled "Cationic Epoxide-Amine Reaction Products", Ser. No. 256,747, filed Apr. 23, 1981, U.S. Pat. No. 4,339,369. The cationic resins are made by reacting an epoxy resin, a dihydric phenol and a tertiary amine which contains one primary hydroxyl group. Such resins are further modified with up to 25 weight percent of a monocarboxylic acid which can be saturated or unsaturated. These cationic coating compositions are not air-drying but are crosslinked by being heated with crosslinking agents.
Another water reducible coating composition is described in U.S. Pat. No. 3,468,779. Epoxy resin esters, i.e., epoxy resins esterified with unsaturated fatty acids, are emulsified in water using as the emulsifying agent a salt of a tertiary amine and a carboxylic acid. Coatings made from such compositions are deposited on metal substrates by electrodeposition and are cured by heating.